Anterior cervical plates can be used for fixation of the cervical spine. Some of the first anterior cervical plate systems used simple plates and screws, such as the Caspar plate system commercially available from Aesculap Inc., of Center Valley, Pa. However, these plates did not have any mechanism for locking the bone screws to the plate, which thereby created a potential situation where one or more of the screws connecting the plate to the spine could back out of its opening in the spine. These systems have subsequently been modified and now most anterior cervical plating systems available in the market employ some kind of screw locking mechanism or anti-screw back out mechanism to prevent the screw from backing out of the bone and plate. They also allow the screws to move relative to the plate either in rotation, translation, or a combination of both rotation and translation.
Some cervical plate systems use a locking element that is engaged once the screw has been inserted into the bone. One example of such a system is commonly known in the industry as a “Codman plate”, which is available from Johnson & Johnson of New Brunswick, N.J. The Codman plate generally includes a cam built into a plate, which can be rotated 180 degrees to engage the head of the screw and prevent it from backing out. This cam only prevents the screw from backing out, and does not prevent it from moving or rotating relative to the plate. Another example of a cervical plate system that includes a locking element is the ABC system, which is commercially available from Aesculap Inc., of Center Valley, Pa. With this system, once a bone screw has been inserted into the bone, a tiny locking screw is advanced into the head of the screw, thereby making the head expand and engage the hole in the plate to prevent the screw from backing out of the plate. In the Vuelock system, which is commercially available from Biomet, Inc. of Warsaw, Ind., a retaining ring that is incorporated into the plate snaps into place once a bone screw is fully inserted, thereby preventing it from backing out of the plate.
In other cervical plate systems, drill guides align screw holes relative to a cervical plate, but screw insertion is performed manually. Since the screws used in cervical plating are typically quite small (e.g., around 4 mm in diameter), it is difficult to determine by feel when the screw is adequately tightened and/or when it has reached its maximum seating depth. Thus, there is a risk with these systems of over-tightening the screw into the bone, which can cause the threads to strip the bone and thereby lose their thread purchase.
The Window cervical plate system, which is commercially available from A-Spine Inc. of Taipei, Taiwan, generally includes a bone screw that engages or meshes with the cervical plate. The cervical plate of the Window system includes a longitudinal slot that corresponds to each bone screw and which meshes with the bone screw threads. The plate itself does not have threads, so meshing with the bone screw is achieved through a thinned out portion of the slot being “captured” between the head and the threaded portion of each bone screw. Relative motion between the plate and the bone screw is mainly translational and possibly includes limited rotation/angulation. A cervical plate system with these general features is also described in U.S. Pat. No. 6,458,122, titled “Spinal Fixation and Retrieval Device”. There is a need, however, to provide an improved cervical plate system that provides flexible rotation capabilities for the screws and that prevents bone screws from backing out once the system is implanted.